Photovoltaic device

ABSTRACT

A method to improve CdTe-based photovoltaic device efficiency is disclosed. The CdTe-based photovoltaic device can include oxygen or silicon in semiconductor layers.

CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/177,502, filed on May 12, 2009, which is incorporated byreference in its entirety.

TECHNICAL FIELD

This invention relates to a photovoltaic device with improvedefficiency.

BACKGROUND

During the fabrication of photovoltaic devices, layers of semiconductormaterial can be applied to a substrate with one layer serving as awindow layer and a second layer serving as the absorber layer. Thewindow layer can allow the penetration of solar radiation to theabsorber layer, where the optical power is converted into electricalpower. Some photovoltaic devices can use transparent thin films that arealso conductors of electrical charge. The conductive thin films caninclude transparent conductive layers that contain a transparentconductive oxide (TCO), such as fluorine-doped tin oxide, aluminum-dopedzinc oxide, or indium tin oxide. The TCO can allow light to pass througha semiconductor window layer to the active light absorbing material andalso serve as an ohmic contact to transport photogenerated chargecarriers away from the light absorbing material. A photovoltaic devicecan have a Cadmium Sulfide (CdS) layer as a semiconductor window layerand a Cadmium Telluride (CdTe) layer as a semiconductor absorber layer.A back electrode can be formed on the back surface of a semiconductorlayer. The back electrode can include electrically conductive material,such as metallic silver, nickel, copper, aluminum, titanium, palladium,chrome, molybdenum or any practical combination thereof.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a photovoltaic device having multiplesemiconductor layers and a metal back contact.

DETAILED DESCRIPTION

A photovoltaic device can include a transparent conductive oxide layeradjacent to a substrate and layers of semiconductor material. The layersof semiconductor material can include a bi-layer, which may include ann-type semiconductor window layer, and a p-type semiconductor absorberlayer. The n-type window layer and the p-type absorber layer may bepositioned in contact with one another to create an electric field.Photons can free electron-hole pairs upon making contact with the n-typewindow layer, sending electrons to the n side and holes to the p side.Electrons can flow back to the p side via an external current path. Theresulting electron flow provides current which, combined with theresulting voltage from the electric field, creates power. The result isthe conversion of photon energy into electric power.

Techniques are developed to extend pn-junction depletion width inCdTe-based photovoltaic device devices. With this approach, highefficiency photovoltaic devices can be achieved through improvedcollection efficiency and thus higher J_(sc). Methods to extendpn-junction depletion width in CdTe include but not limited to modifyingfilm properties in the p- and/or n-type layer. For example it can beachieved by adding certain type of materials to CdTe and/or n-typewindow layer. The additive materials can include such as but not limitedto silicon-containing materials. Such additions can be introducedthrough raw material treatment, during coating process orpost-deposition treatment. It is also found that some of these materialsmay behave as flux agent modifying CdTe crystalline growth. Modificationin sub band-gap optical properties is also observed.

In one aspect, a method of manufacturing a photovoltaic device caninclude the steps of depositing a transparent conductive oxide layeradjacent to a substrate, depositing a semiconductor window layeradjacent to the transparent conductive oxide layer, depositing asemiconductor absorber layer adjacent to the semiconductor window layer,and depositing a back contact layer adjacent to the semiconductorabsorber layer. One or more of the semiconductor window layer and thesemiconductor absorber layer can include a carrier concentrationmodifier. One or more of the semiconductor window layer and thesemiconductor absorber layer can include a II-VI semiconductor. Thesemiconductor window layer can include cadmium sulfide (CdS). Thesemiconductor absorber layer can include cadmium telluride (CdTe). Thecarrier concentration modifier can include oxygen. The carrierconcentration modifier can include silicon. The method can furtherinclude increasing carrier concentration in the semiconductor windowlayer.

The method can further include decreasing carrier concentration in thesemiconductor absorber layer. The method can further include addingmaterial comprising silicon to the semiconductor absorber layer. Themethod can further include adding material comprising oxygen to thesemiconductor absorber layer. The method can further include addingmaterial comprising oxygen to the semiconductor window layer. The methodcan further include adding material comprising oxygen to thesemiconductor absorber layer and semiconductor window layer. The step ofdepositing of the semiconductor absorber layer can include vaportransport deposition (VTD) from a cadmium and telluride source.

The vapor transport deposition (VTD) can include blending materialcomprising oxygen with the cadmium and telluride source. The vaportransport deposition (VTD) can include blending material comprisingsilicon with the cadmium and telluride source. The step of depositing ofthe semiconductor absorber layer can include using precursor comprisingoxygen. The step of depositing of the semiconductor absorber layer caninclude using precursor comprising silicon. The method can furtherinclude a step of post-deposition treatment on the semiconductorabsorber layer and semiconductor window layer in an environmentincluding oxygen. The method can further include a step ofpost-deposition treatment on the semiconductor absorber layer andsemiconductor window layer in an environment including silicon. Themethod can further include a step of depositing of an additional layerincluding oxygen, wherein the oxygen of the additional layer can diffuseinto the semiconductor absorber layer and semiconductor window layer.The method can further include a step of depositing of an additionallayer comprising silicon, wherein the silicon of the additional layercan diffuse into the semiconductor absorber layer and semiconductorwindow layer.

The transparent conductive oxide layer can include oxygen, wherein theoxygen of the transparent conductive oxide layer can diffuse into thesemiconductor absorber layer and semiconductor window layer. Thetransparent conductive oxide layer can include silicon, wherein thesilicon of the transparent conductive oxide layer can diffuse into thesemiconductor absorber layer and semiconductor window layer. The backcontact layer can include oxygen, wherein the oxygen of the back contactlayer can diffuse into the semiconductor absorber layer andsemiconductor window layer. The back contact layer can include silicon,wherein the silicon of the back contact layer can diffuse into thesemiconductor absorber layer and semiconductor window layer.

The method can further include adding material comprising a group IVelement to the semiconductor absorber layer. The method can furtherinclude adding material including a group IV element to thesemiconductor window layer. The method can further include addingmaterial including a group IV element to the semiconductor absorberlayer and semiconductor window layer. The method can further includeadding material comprising a group VI element to the semiconductorabsorber layer. The method can further include adding materialcomprising a group VI element to the semiconductor window layer. Themethod can further include adding material comprising a group VI elementto the semiconductor absorber layer and semiconductor window layer.

In one aspect, a CdTe-based photovoltaic device can include atransparent conductive oxide layer, a semiconductor bi-layer adjacent tothe transparent conductive oxide layer, and a back contact layeradjacent to the semiconductor bi-layer. The semiconductor bi-layer caninclude a semiconductor absorber layer and a semiconductor window layerand one or more of the semiconductor window layer and the semiconductorabsorber layer can include a carrier concentration modifier. Thesemiconductor window layer can include increased carrier concentration.The semiconductor absorber layer can include decreased carrierconcentration. The semiconductor absorber layer can include materialincluding silicon. The semiconductor absorber layer can include materialincluding oxygen. The semiconductor window layer can include materialincluding oxygen. The semiconductor window layer and semiconductorabsorber layer can include material including oxygen.

Referring to FIG. 1, a photovoltaic device 100 can include a transparentconductive oxide layer 120 deposited adjacent to a substrate 110.Transparent conductive oxide layer 120 can be deposited on substrate 110by sputtering or evaporation. Substrate 110 can include a glass, such assoda-lime glass. Transparent conductive oxide layer 120 can include anysuitable transparent conductive oxide material, including a cadmiumstannate, an indium-doped cadmium oxide, or a tin-doped indium oxide. Asemiconductor bi-layer 130 can be formed or deposited adjacent toannealed transparent conductive oxide layer 120. Semiconductor bi-layer130 can include semiconductor window layer 131 and semiconductorabsorber layer 132. Semiconductor window layer 131 of semiconductorbi-layer 130 can be deposited adjacent to annealed transparentconductive oxide layer 120. Semiconductor window layer 131 can includeany suitable window material, such as cadmium sulfide, and can be formedby any suitable deposition method, such as sputtering or vapor transportdeposition. Semiconductor absorber layer 132 can be deposited adjacentto semiconductor window layer 131. Semiconductor absorber layer 132 canbe deposited on semiconductor window layer 131. Semiconductor absorberlayer 132 can be any suitable absorber material, such as cadmiumtelluride, and can be formed by any suitable method, such as sputteringor vapor transport deposition. Back contact 140 can be depositedadjacent to semiconductor absorber layer 132. Back contact 140 can bedeposited adjacent to semiconductor bi-layer 130. A back support 150 canbe positioned adjacent to back contact 140. A photovoltaic device canhave a Cadmium Sulfide (CdS) layer as a semiconductor window layer and aCadmium Telluride (CdTe) layer as a semiconductor absorber layer.

In CdTe-based photovoltaic device devices, the depletion region occursmostly within the CdTe because it is typically less doped as compared tothe n-type layer. The extension of the depletion width increases thedepth from which photo-generated carrier can be efficiently collected,which improves photocurrent and solar efficiency. As the results, onewill expect to see higher total current, overall higher quantumefficiency throughout the solar spectrum. There are multiple approachesled to extended depletion width. For example, the depletion widthtypically increases when free carrier concentration in CdTe decreases.This can be achieved by reducing the concentration of doping levels inthe CdTe layer or by introducing compensating levels within the energygap of the semiconductor. While introducing compensating levels couldpotentially lower device open circuit voltage and fill factor throughadditional recombination through these levels, we have found in somecases that the improvement in close-circuit current density (J_(sc))dominates, thus to result in higher efficiency devices. Close-circuitcurrent density (J_(sc)) is the photocurrent output from a photovoltaicdevice when the output terminals are short-circuited. For example,adding silicon is found to be very effective to induce such efficiencyimprovement. The earlier approach of silicon addition utilized elementsfrom vapor transport deposition (VTD) tools. These elements includingsilicon, which can be released thermally or when reacting with otherreactants including O₂, H₂, H₂O and even ambient air. The releasedsilicon was included in the as-deposited film and can be detected bycompositional analysis techniques such as secondary-ion massspectroscopy (SIMS) or ICP-MS. Note this approach can be easilyimplemented at other deposition tools in addition to VTD.

In one example, Si can be released from VTD elements and result inhigher efficiency through improved J_(sc). When Si was forced to releasefrom one coater, the efficiency can be improved and was driven byJ_(sc). Compared to IV parameters from control samples runningsimultaneously on another VTD tool but without forced Si release, J_(sc)and efficiency of the samples with forced Si release can be about 4%higher. Si-doping concentration can be in the range from 1×10¹⁶ to1×10¹⁸/cm³.

The addition of oxygen during VTD process can also yield high J_(sc)devices. A typical approach of doing so is introducing oxygen-containingsource during deposition. By delivering oxygen into VTD chamber duringfilm growth, an improvement on J_(sc) can be achieved. The secondary-ionmass spectroscopy (SIMS) can reveal the actual oxygen incorporation inCdTe.

Direct addition of materials including silicon during VTD process isalso very effective in producing high J_(sc) devices. In one example,this is realized through mixing silicon-containing powders with CdTepowders to a certain blend ratio. Incorporation of Si in material can bemeasured by secondary-ion mass spectroscopy (SIMS). High J_(sc) devicescan be achieved by this method, and under certain doping conditions,higher efficiency devices can be realized (such as 850 ppm). The blendpercentage of Si can be in the range from 0.001% to 1%, or 0.01% to0.2%. J_(c) and efficiency of the samples with the incorporation of Sican be about 1%, 2%, 3%, 4%, or 5% higher. In one example, silicon canmodify the growth of CdTe and the crystalline structures. With theinclusion of silicon, grain size of CdTe becomes significantly smaller,while the crystalline orientation changes from (001), without Siaddition, to (011) with modest Si addition, and to (012) with heavy Siaddition. The improvements in optical transmission, reflectance andabsorbance with silicon-inclusion can also be found. In general it isfound that within the energy band gap, the transmission and reflectionwill decrease, and absorbance will increase with the increase of siliconconcentration in the film. The inclusion of Si-containing material canlead to a measureable change in sub-band gap absorption (800-1400 nm).

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Itshould also be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention.

What is claimed is:
 1. A method of manufacturing a photovoltaic devicecomprising the steps of: depositing a transparent conductive oxide layerover a substrate; depositing a semiconductor window layer over thetransparent conductive oxide layer; depositing a semiconductor absorberlayer over the semiconductor window layer, wherein one or more of thesemiconductor window layer and the semiconductor absorber layercomprises a carrier concentration modifier, wherein the carrierconcentration modifier comprises silicon; and depositing a back contactlayer over the semiconductor absorber layer.
 2. The method of claim 1,wherein one or more of the semiconductor window layer and thesemiconductor absorber layer comprises a II-VI semiconductor.
 3. Themethod of claim 1, wherein the semiconductor window layer comprisescadmium sulfide (CdS).
 4. The method of claim 1, wherein thesemiconductor absorber layer comprises cadmium telluride (CdTe).
 5. Themethod of claim 1, further comprising increasing carrier concentrationin the semiconductor window layer.
 6. The method of claim 1, furthercomprising adding material comprising silicon to the semiconductorwindow layer or the semiconductor absorber layer.
 7. The method of claim1, wherein the step of depositing of the semiconductor absorber layercomprises vapor transport deposition (VTD) from a cadmium and telluridesource.
 8. The method of claim 7, wherein the vapor transport deposition(VTD) comprises blending material comprising silicon with the cadmiumand telluride source.
 9. The method of claim 8, wherein a blendpercentage of silicon in the cadmium and telluride source is in therange of 0.01% to 0.02%.
 10. The method of claim 8, wherein a blendpercentage of silicon in the cadmium and telluride source is in therange of 0.001% to 1%.
 11. The method of claim 1, wherein the step ofdepositing of the semiconductor absorber layer comprises using aprecursor comprising silicon.
 12. The method of claim 1, furthercomprising a step of post-deposition treatment of the semiconductorabsorber layer and semiconductor window layer in an environmentcomprising silicon.
 13. The method of claim 1, further comprising a stepof depositing of an additional layer comprising silicon, wherein thesilicon of the additional layer can diffuse into the semiconductorabsorber layer and semiconductor window layer.
 14. The method of claim1, wherein the transparent conductive oxide layer comprises silicon,wherein the silicon of the transparent conductive oxide layer candiffuse into the semiconductor absorber layer and semiconductor windowlayer.
 15. The method of claim 1, wherein the back contact layercomprises silicon, wherein the silicon of the back contact layer candiffuse into the semiconductor absorber layer and semiconductor windowlayer.
 16. The method of claim 1, further comprising decreasing carrierconcentration in the semiconductor absorber layer.